Geschichte: During a hiking tour along Waikato River, I didn't realise that I was still in the caldera of Lake Taupo. When I suddenly found a rock face full of thin laminated volcanic material I collected a small hand specimen to develop theories about how its formation. Back to Zürich at ETH we discussed this and I was explained that although it looked like a sedimentary rock, it's actually an obsidian.
Geologie: This rock sample is an obsidian (black) with pumice layers (white). Obsidian is a typical volcanic rock, also known as 'volcanic glass'. The movement of lavaflow created the different layers of obsidian and pumice. Bubbles of gas in the lava foamed up parts of the obsidian so that the it became lighter pumice. However, the chemical composition of the two layers remained the same.